1. Field of the Invention
The present invention relates to logic communication networks formed of transceiver equipment and more particularly networks for data packet multipoint communication. In such a communication mode, each transceiver may communicate with each of the other transceivers of the network, by transmitting data packets over a data transmission carrier common to all the transceivers. Each data packet contains the address of its addressee, similarly to a letter or a packet in the postal network.
2. Description of the Prior Art
Numerous types of data communication networks are known of the multipoint packet type. The data transmission carrier is formed of a network of one or more transmission media, such as coaxial cables, optical fibers or others, connected together and permanently to a plurality of transceivers. The transceivers may be all purpose computers, specialized computers (micro-processors), devices such as input-output stations or distant terminals or different other peripheral members.
One of the main functions which a multipoint packet type data transmission network of probabilist type must provide is the management of collisions. The data transmission carrier can in effect generally transmit only a single packet at a time. If two transmitters decide to send a message simultaneously, collision occurs in the data transmission carrier, so that the data is not received correctly. To manage these collisions, networks are already known using the principle of contention. According to this principle, each transceiver connected to the network can only transmit if no other transmission is in progress.
The high speed communication needs between computers have led to developing networks using synchronous transmission techniques, working for example at speeds of several megabauds. In these techniques, coding of the signals generally makes it possible to regenerate the clock at reception, i.e. the transmitted data conveys the clock signal of the sender, and this clock signal is used by the addressee for sampling the bits received and deserializing the information. In this type of connection, all the characters are transmitted following each other, without dead times and the beginning of the transmission is generally characterized by sending one or a succession of bits used by the receiver for synchronizing itself. Any interruption in transmission causes desynchronization of the addressee.
The document FR-A-2 306 478 describes such a synchronous transmission network of packet multipoint type. In this device, a message transmission begins by a synchronization bit. An interruption at any time is interpreted as the end of the message, the network placing itself immediately in the rest condition. This device operates with a code conveying the clock of the sender. The messages contain the addresses of the sender and of the addressee. Should a collision occur in the network, the senders interrupt themselves and take up transmission again after a wait of random duration, this duration being possibly practically zero.
The document EP-A-0023105 also describes such a device using the contention principle, but in which the message is transmitted in a particular code containing packets of eight bits. The transmission is of synchronous type, the message conveying the clock. No addressee's address is sent, so that all the receivers are addressees of all the messages. The particular code makes it necessary to wait at least the duration of a packet before taking up an interrupted transmission again.
The document GB-A-2 013 452 describes a determinist type device in which the sending equipment designates its successor, the different equipment being thus enabled to transmit one after the other in a modifiable order. It is a question of a technique very different from contention.
For communication needs, less rapid than the synchronous techniques, and leading to less expensive circuits, data communication networks have been developed of multipoint packet asynchronous type. Such a technique is described in the document WO 82/04366. In such asynchronous type transmission networks, making it possible to construct installations with lower flowrate but much less expensive than synchronous techniques, detection of the availability of the network is more delicate. In fact, in such an asynchronous mode transmission, the messages are formed of a succession of characters separated by intervals during which no signal is transmitted, so that it is insufficient to check the absence of signals to know if a message is not being transmitted.
To solve this problem, the document WO 82/04366 describes a method and device in which the message, formed of a succession of asynchronous characters, is conditioned by each transmitter which inserts, before the first character of the message, a particular beginning of message signal, and which inserts, after the last character of the message, a particular end of message signal. On each character or signal transmission, the sender compares the signals corresponding to the characters transmitted and the signals detected simultaneously on listening to the network; if the signals are identical, it continues the transmission of the message; if the signals are different, it transmits an end of message character and it immediately interrupts the transmission of the message, and remains listening for a random time greater than a minimum waiting time. In this document, the beginning of message and end of message signals are interruption characters such as the interruption characters currently used in communication techniques of asynchronous type. These interruption characters comprise a continuous signal of a duration appreciably greater than the duration of the characters forming the message, so that their form is very different from all the codes used in the characters forming the message.
Such a technique makes it necessary to provide, in each transmitter, special circuits for generating the interruption signal at the beginning and end of message; it further requires having, in each receiver, special circuits for detecting the interruption signals. These special circuits are not normally required for simple transmission of messages over an asynchronous network. The result is a relative complication of the circuits.
Furthermore, the technique described in this document makes it necessary to wait, at the end of message, for a time greater than the interruption character duration, before beginning a new transmission again. Since the interruption characters are relatively long, the result is a substantial loss of transmission time.